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TROPICAL STORM IVAN DISCUSSION NUMBER 12
NATIONAL WEATHER SERVICE MIAMI FL
5 AM EDT WED SEP 23 1998
THE INITIAL INTENSITY WILL REMAIN AT 45 KNOTS BASED UPON DVORAK
CLASSIFICATIONS. THE SYSTEM CONTINUES TO UNDERGO SHEARING FROM THE
UPPER-LEVEL TROUGH TO THE WEST. THE INTENSITY WILL BE HELD AT 45
KNOTS THROUGH 72 HOURS AS IN THE PREVIOUS PACKAGE. THE SYSTEM MAY
BECOME EXTRA-TROPICAL BEFORE THE END OF THE PERIOD.
THE INITIAL MOTION ESTIMATE IS NOW 335/13 KNOTS. THE PRIMARY
STEERING MECHANISM FOR THIS TROPICAL CYCLONE WILL BE THE TROUGH
TO THE WEST OF IVAN. MOST OF THE 00Z TRACK MODELS SHOW A
NORTH-NORTHWEST OR NORTH MOTION THROUGH 36 HOURS WITH A TURN TOWARD
THE NORTHEAST. HOWEVER...BECAUSE OF THE DIFFERENCES IN THE FORWARD
SPEEDS OF THE VARIOUS TRACK MODELS THE TURN TOWARD THE NORTHEAST
OCCURS AT DIFFERENT LATITUDES RESULTING IN A DIVERGENT TRACK SUITE.
THE OFFICIAL TRACK FORECAST FOLLOWS THIS BASIC SCENARIO AND IS
SIMILAR THE PREVIOUS PACKAGE.
JARVINEN
FORECAST POSITIONS AND MAX WINDS
INITIAL 23/0900Z 27.3N 38.1W 45 KTS
12HR VT 23/1800Z 29.2N 38.9W 45 KTS
24HR VT 24/0600Z 31.4N 39.7W 45 KTS
36HR VT 24/1800Z 33.5N 40.1W 45 KTS
48HR VT 25/0600Z 35.4N 39.9W 45 KTS
72HR VT 26/0600Z 38.9N 38.4W 45 KTS
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